The present invention relates to a method of detecting a geological discontinuity present in a medium which may be represented in the form of a seismic block or on the basis of several seismic sections of the said medium.
Regardless of the type of seismic campaign in two dimensions (2D) or three dimensions (3D), use is made of one or more wave-emitting sources which are arranged at the surface of the said medium, and receivers, which are likewise arranged at the surface of the said medium and which receive and record the elastic waves reflected off each reflector constituted by the interface between two geological formations of a different nature, the said surface possibly being the surface of the ground or of the sea. The set of recordings obtained makes it possible to produce a representation or image of the medium in 2D or 3D. In general, interference, multiple reflections or artefacts introduce noise, indeterminations, which sometimes render the image of the sub-surface difficult to interpret even after more or less complex processing. Therefore, interpreters do not always succeed in accurately charting and positioning the sought-after structures such as discontinuities constituted in particular by faults, channels, etc. To improve the interpretation of seismic images, specialists interpolate the reliable seismic events detected in neighbouring planes, using in particular the process known as the process of continuity criteria.
When a seismic block representative of the medium to be explored is available it is common to produce an animated display of the seismic block; coherent events, which have some spatial and temporal extent, are distinguished from the surrounding noise, thereby making it possible to eliminate some detection and positioning uncertainties. This is because, during this display, the human eye, via the remanence effect, keeps the image displayed on the retina for a certain time, thereby enabling it to analyse several images of a sequence simultaneously. When a relevant configuration or relevant seismic event, which is visible in an image, is merely an negligible accident, it may be assumed that it will rapidly disappear; conversely, if this configuration represents a fraction of a noteworthy event, then there is a high chance of finding it also in the neighbouring images. Thus, the temporal continuity of events is perceived locally and reliably by the human eye; this continuity never so to speak being oriented in the direction of display, an impression of displacement, of movement, results at every point.
Calculations of optical flow for certain images have been proposed, for example for a conventional sequence of images in which there is just a single direction of intelligible display. Such calculations are defined for example in xe2x80x9cInverse perspective mapping simplifies optical flow computation and obstacle detectionxe2x80x9d by H. A. MALLOT et al., Biological Cybernetics, 64, 1991, pages 177-185; or in xe2x80x9con the estimation of optical flow: relations between different approaches and some new resultxe2x80x9d by H. H. NAGEL, Artificial Intelligence, 33, 1987, pages 299-324. The term optical flow is understood to mean a spatial distribution of the apparent velocities which are observed during the animation of a sequence of images. The optical flow, which at every point provides the displacement observed, is therefore one possible representation of dynamic perception.
It should however be noted that such a conventional sequence of images represents a solid, undeformable world in which the concepts of objects, of void and of transparency are omnipresent; moreover, the only direction of intelligible display is the direction of acquisition.
Among the methods which use the calculation of optical flow and are applied to non-seismic images, mention may be made of those which compensate for the lack of local information by interpolating the velocity estimates made at contour level (K. Y. WOHN et al. xe2x80x9cA contour-based recovery of image flow: iterative transformation methodxe2x80x9d: IEEE Transactions on Pattern Analysis and Machine intelligence, vol. 13, No. 8, 1991, pages 746-760) or directly over large-size regions (D. P. KOTTLE et al., xe2x80x9cMotion Estimation via Cluster Matchingxe2x80x9d: IEEE transaction on Pattern Analysis and Machine Intelligence, vol. 16, No. 1, Nov. 1994, pages 1128-1132). These methods are inaccurate and require expensive prior processing such as contour detection or segmentation into regions. Moreover, the basic images processed via the optical flow are of reduced dimensions.
In a sequence of seismic images, the data are said to be xe2x80x9cfullxe2x80x9d since they represent a part of the interior of a solid, in this instance the sub-surface. In data of these types, all the above concepts are abandoned, there no longer being any void and it being possible for space to be viewed in all directions. Such isotropy provides for the possibility of an infinity of different observations and therefore of a considerable amount of information.
The aim of the present invention is to quantify the dynamic perception given by the calculation of an optical flow of an image by applying it to one or more seismic images so as to accentuate three-dimensional events which are difficult to perceive in a fixed plane and thus ease the work of interpreters.
Seismic images exhibit a textured character by comparison with images consisting of wide monochrome regions, and therefore, the local variations in intensity are relevant at every point. The methods recalled above are ill-suited to the calculation of the optical flow of seismic images.
A subject of the present invention is a method of detecting a geological discontinuity in a seismic block, characterized in that it consists in:
selecting within the seismic block at least one section comprising at least one discontinuity, the said section constituting a seismic image,
calculating a raw optical flow over the said seismic image so as to obtain a first representation of the optical flow, in which the discontinuity constitutes a moving front, and in
smoothing the said representation of the optical flow by the unsupervised process of dynamic clusters so as to obtain a second representation of the optical flow, in which the said discontinuity exhibits enhanced contrasts.
An advantage of the present invention lies in the fact that the selected smoothing makes it possible to attribute the velocity of highest probability, that is to say the one best represented locally, to each pixel of the basic seismic image.
Smoothing by dynamic clusters leads to relevant results regardless of the spatial configuration of the optical flow and in an unsupervised manner. The conjugate use of optical flow and smoothing by dynamic clusters makes it possible to detect precise and continuous boundaries.
Other advantages and characteristics will emerge more clearly from reading the description of the method according to the invention applied to a discontinuity or seismic event constituted by a fault, as well as the appended drawings in which:
FIG. 1 is a seismic section of the sub-surface of the medium explored;
FIG. 2 is an image or representation of the optical flow calculated over the seismic image of FIG. 1,
FIG. 3 is a representation of the image of the optical flow of FIG. 2, after smoothing,
FIG. 4 is a representation of the variance of the smoothed optical flow, this variance being superimposed on the seismic section of FIG. 1,
FIGS. 5 to 8 partially represent the seismic section of FIG. 1 over which a calculation window is shifted,
FIGS. 9 to 12 represent the spread of the clusters of points corresponding to FIGS. 5 to 8 respectively,
FIG. 13 represents the behaviour of the components U and V of the velocity obtained following average smoothing along a trajectory P1-P4,
FIG. 14 represents the behaviour of the components U and V of the velocity obtained following smoothing by dynamic clusters along the same trajectory P1-P4.